In recent years, industrial robot control technology has advanced with advancement in computer technology. Accordingly, industrial robots are required to be able to operate in a more complicated and sophisticated manner, and required to be faster and more precise. For example, in the case of conventional six-axis articulated robots widely used as industrial robots, although conventional six-axis articulated robots are sufficiently capable of moving to a specified position, it is difficult for them to perform complex tasks while efficiently avoiding various obstacles existing in their moving space. Therefore, in recent years, development of seven-axis articulated robots has been actively conducted. A seven-axis articulated robot is configured such that a six-axis articulated robot including a total of six rotational axes at respective joints JT1 to JT6 additionally includes a redundant rotational axis at a joint JT7.
In a method of controlling a seven-axis articulated robot, similar to a conventional method of controlling a six-axis articulated robot, inverse transformation (coordinate transformation) with which to obtain each joint angle (joint coordinates) from a positional orientation of the wrist (wrist coordinates) of the robot is performed. In order to realize smooth motion similar to motion of a human arm, the angle of the elbow of the robot needs to be a constant angle as a constraint condition in performing the inverse transformation.
With reference to FIG. 10, a description is given by taking as an example a seven-axis articulated robot including a mechanism in which the rotational axis of a joint JT1, the rotational axis of a joint JT2, and the rotational axis of a joint JT7 intersect with each other at one point.
FIG. 10 defines a “shoulder (S)”, an “elbow (E)”, and a “wrist (W)” of the seven-axis articulated robot. The shoulder (S) indicates an intersection point where the rotational axes of the respective joints JT1, JT2, and JT7 intersect with each other; the wrist (W) indicates an intersection point where the rotational axes of respective joints JT4, JT5, and JT6 intersect with each other; and the elbow (E) indicates the rotational axis of a joint JT3. As shown in FIG. 11, a plane SEW defined by the shoulder (S), elbow (E), and wrist (W), i.e., defined by these three points, can be rotated around a straight line SW with the position and orientation of the wrist kept constant. Generally speaking, an elbow angle θE is defined as a rotating joint angle ∠EHE′ of the “elbow (E)” around a vector connecting the shoulder (S) and the wrist (W). Inverse transformation is performed under a constraint condition that the elbow angle θE is kept constant.
For example, Patent Literature 1 discloses in paragraph [0012] that a seven-axis articulated robot can be considered to be equivalent to a six-axis articulated robot by fixing a seventh joint (joint axis J3) of the seven-axis articulated robot. Patent Literature 1 also discloses in paragraph [0014] that a seven-axis articulated robot with no Y-direction offset between joint axes J1 and J2 is configured such that, in order to avoid interference with a six-axis articulated robot installed at a predetermined distance from the seven-axis articulated robot, an elbow (joint axis J4) moves in a manner to draw a substantially arc trajectory while using an XY plane in which the joint axis J2 is included, a shoulder (intersection point of the joint axes J1 and J2), and a wrist (joint axis J6) as supporting points.